And of course the Darwin book is available to us all for free at google books. At around page 93 he gets into it, explaining (with examples) how in societies of the most primitive sort and onward, fear of social approbation works to the common good, countering the instinct for self-preservation. Then he claims:

"As man advances in civilization, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all members of the same nation, though personally unknown to him. This point once being reached, there is only an artificial barrier to his sympathies being extended to the men of all nations and races."


http://books.google.com/books?id=iArG1dDytFAC&printsec=frontcover&dq=The+Descent+of+Man#PPA134,M1